A NASA spacecraft science

There ought to be plenty of fireworks this Fourth of July. But the best pyrotechnics show is likely to take place 268 million miles away, when a probe fired from the Deep Impact spacecraft is scheduled to collide with Comet Tempel 1, a nine-mile-long rock roaring through space at 66,880 m.p.h. The planned cosmic crack-up will gouge out a football-field-size crater and may be visible from the U.S. Pacific Coast and points west. It may also reveal a lot about the chemistry of comets, fossils of the early solar system.

Deep Impact is really a craft and a half: an SUV-size mother ship and a smaller impactor--basically a guided missile equipped with a targeting system and a camera. Early on July 3, when the paired machines are 24 hours from the comet, the mother ship is set to release the impactor to free-fly the last 500,000 miles. If all goes well, the probe and comet should collide at 1:52 a.m. E.T. on July 4, producing the same explosive bang as 4.5 tons of TNT. The mother ship, trailing a safe 310 miles behind, will photograph the crater and analyze the debris field thrown up.

There's no danger that the distant detonation will send the comet tumbling our way. The probe will be like a bug on Tempel 1's windshield, changing its speed by 0.014 in./hr. --With reporting by Dan Cray/Pasadena

[The following text appears in a complex diagram.]

The Mission

1 The SUV-size probe began its journey from Earth last January carrying an 820-lb. impactor

•Sun

•Earth at launch

•Path of spacecraft

•Tempel-1 orbit

•Impact July 4

•Earth at impact

2 The spacecraft first detects the comet when it’s still 60 days away. The ship begins refining its aim, studying the comet’s rotation and ambient dust

3 The impactor is released 24 hours before the planned collision and travels the final 500,000 miles on its own, adjusting its trajectory as it goes

4 The impactor strikes the comet at a relative speed of 23,000 m.p.h., annihilating itself and gouging out a crater that could be up to 14 stories deep

5 The mother ship is programmed to send back 800 seconds’ worth of pictures of the impact, the crater, and the glowing debris field thrown out

The Flyby Spacecraft The ship, 11 ft. long and weighing 1,325 lbs., is powered by solar panels and a rechargeable battery. It uses a suite of instruments to survey the comet. It also gathers and relays data from the impactor

•High-resolution Instrument Will take detailed pictures of the inside of the crater

•High-gain Antenna For high speed data transfer to Earth

•Solar Panels

•Star trackers

•Medium-resolution instrument Will collect wideview images of the material ejected from the crater

•Telescopes

The Impactor Designed to collide and die, it carries few instruments but a lot of weight 820 lbs. in all. More than 200 of those pounds make up a copper cratering mass, which increases the size of the hole the impactor digs

•Star tracker

•Antenna

•Targeting sensor Can collect images of the comet until 3 sec. before impact